Biodiesels produced from vegetable oils have drawn much attention as alternate fuels for fossil fuels and also in terms of a small discharge amount of carbon dioxide, and therefore, an increase in demand for biodiesels has been expected. Since the production of such biodiesels is accompanied by the formation of glycerin as a by-product, it is required to make effective use of glycerin. An embodiment of making use of glycerin is to use glycerin as a raw material for acrolein, and acrolein is further used as a raw material of acrylic acid.
Japanese Patent Laid-open Publication (Kokai) No. 2005-213225 discloses two processes for producing acrylic acid by oxidation of glycerin. The first process disclosed in this publication is a process for producing acrylic acid by producing a gaseous substance with gas-phase dehydration of glycerin and carrying out oxidation of the gaseous substance. That is, this first process is a two-step reaction process comprising dehydration of glycerin and oxidation of acrolein, both of which occur in different reaction systems. The second process is a process for producing acrylic acid from glycerin using a mixed catalyst composed of a catalyst for dehydration and a catalyst for gas-phase oxidation or using a catalyst having both the function of a catalyst for dehydration and the function of a catalyst for gas-phase oxidation. That is, the second process is a one-step reaction process comprising dehydration of glycerin and oxidation of acrolein, both of which occur in the same reaction system. As specific examples of the catalysts to be used in the above first process, there are disclosed catalysts for dehydration carrying phosphoric acid supported on an α-alumina carrier; and catalysts for oxidation which are Mo—V—W—Cu type composite oxides. Further, as specific examples of the catalyst to be used in the above second process, there are disclosed mixed catalysts composed of catalysts for dehydration carrying phosphoric acid supported on an α-alumina carrier and catalysts for oxidation which are Mo—V—W—Cu type composite oxides; and catalysts carrying Mo—V—W—Cu type composite oxides supported on α-alumina carrying phosphoric acid (i.e., catalysts having both the function of catalysts for dehydration and the function of catalysts for oxidation).
First, the present inventors have proceeded with studies on the above two-step reaction process, and as a result, they have found that when acrylic acid is produced from glycerin under the reaction conditions described in the Japanese Patent Laid-open Publication (Kokai) No. 2005-213225, the performance of a catalyst for oxidation is drastically deteriorated. Further, they have also found that one factor for the deterioration of catalyst performance is phenol and hydroxyacetone, both of which are formed as by-products in the dehydration of glycerin.
When acrylic acid is produced by a two-step reaction process, it is desired to decrease the amounts of by-products such as phenol and hydroxyacetone for the purpose of suppressing the deterioration of catalyst performance for oxidation.
When acrylic acid is produced by a two-step reaction process, it needs a refining step for removing by-products by distillation or any other means from acrolein containing the by-products formed by the dehydration of glycerin in the first step for the purpose of giving a prolonged life of a catalyst for oxidation. However, there are concerns such as loss of acrolein during the refining step; and clogging of pipes, distillation towers, or others with polymers formed during the refining step because of high polymerizability of acrolein. Further, the number of steps for producing acrylic acid from glycerin is desired to be small for the purpose of producing acrylic acid at a lower cost.
By the way, International Publication WO 2006/087084 discloses a process for producing acrolein from glycerin as reaction of the first step in the two-step reaction process. Along with this disclosure, this publication also discloses that phenol, hydroxyacetone, and others are formed as by-products in the production of acrolein and that the amounts of these by-products are decreased if oxygen at an amount lower than the flammability range is allowed to coexist in the dehydration system of glycerin. When oxygen is allowed to coexist in the dehydration system of glycerin, the above by-products can be reduced; however, the reduction of the by-products cannot be said to be sufficient for using the resultant acrolein in the production of acrylic acid without refining.
Then, the present inventors have studied on the one-step reaction process disclosed in the Japanese Patent Laid-open Publication (Kokai) No. 2005-213225. That is, the present inventors have studied on the reaction of directly obtaining acrylic acid from glycerin using a catalyst containing a combination of phosphoric acid and a Mo—V—W—Cu type composite oxide. However, the activity of the catalyst is remarkably lowered and is therefore insufficient from an industrial point of view under the reaction conditions disclosed in the Japanese Patent Laid-open Publication (Kokai) No. 2005-213225.